|Publication number||US7592739 B2|
|Application number||US 10/502,802|
|Publication date||Sep 22, 2009|
|Filing date||Feb 11, 2003|
|Priority date||Feb 13, 2002|
|Also published as||EP1474866A2, EP1474866B1, US20050162040, WO2003069776A2, WO2003069776A3|
|Publication number||10502802, 502802, PCT/2003/427, PCT/FR/2003/000427, PCT/FR/2003/00427, PCT/FR/3/000427, PCT/FR/3/00427, PCT/FR2003/000427, PCT/FR2003/00427, PCT/FR2003000427, PCT/FR200300427, PCT/FR3/000427, PCT/FR3/00427, PCT/FR3000427, PCT/FR300427, US 7592739 B2, US 7592739B2, US-B2-7592739, US7592739 B2, US7592739B2|
|Original Assignee||Commissariat A L'energie Atomique|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (16), Referenced by (4), Classifications (12), Legal Events (2)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This invention relates to a tuneable MEMS (Micro-Electro-Mechanical System) film bulk acoustic micro-resonator.
Several filtering levels are necessary, for example in a transmission-reception system for mobile communications: the antenna output as a band or rejection filter, etc. At the moment, two main solutions are used to make RF filters:
A third line of research appears, based on the use of mechanical Film Bulk Acoustic Resonators (FBARs). These resonators may be integrated and provide high quality factors (more than 1000). Consequently, these are useful devices since they normally offer good performances (high quality factor) and a low production cost (devices that can be integrated).
Therefore acoustic wave resonators are classified in two categories: Surface Acoustic Wave (SAW) resonators and Film Bulk Acoustic Resonators (FBAR).
For SAW resonators, a surface wave is generated by exciting a piezoelectric material (quartz, LiNbO3, LiTaO3) through a comb electrodes network. This wave is detected by a second set of interdigitised electrodes. The width, spacing and number of excitation and detection combs control the resonant frequency of the oscillator or the filter pass band. SAW filters are used in frequency ranges varying from 100 MHz to about 2.5 GHz. The surface area of these devices, which is a few mm2, and the piezoelectric materials used (quartz, LiNbO3, LiTaO3, etc.) make the principle unsuitable for production on an integrated circuit. Although this type of component is high performance and widely tested, it is reserved more particularly for a hybridisation technology.
In the case of FBAR resonators, a bulk acoustic wave is generated by excitation of a thin piezoelectric layer (made of AlN, PZT or ZnO) between two electrodes. For these micro-mechanical resonators, filters are usually made by coupling several of these resonators together. Resonant frequencies of the resonators, which are usually close, fix the central frequency of the filter. The pass band of the filter depends on the coupling characteristics (position, dielectric strength), while the number of resonators gives the order of the filter. This type of resonator can be used to make filters within the 300 MHz-12.5 GHz range.
FBAR resonators can be classified in two groups, called FBAR-MEMS and SMR in the specialised literature. In the case of FBAR-MEMS, the piezoelectric material resonator is separated from the substrate to avoid any dissipation of acoustic waves. This spacing is obtained either by etching the back face of the substrate, or by suspending the resonator by the use of a sacrificial layer. In the case of SMRs (Solidly Mounted Resonator), an acoustic Bragg grating is used underneath the piezoelectric material resonator. This acoustic Bragg grating is composed of a multi-layer of materials, each layer of the multi-layer being λ/4 thick (where λ is the wavelength corresponding to the frequency of the resonator) and having a Young's modulus very different from one layer to the next. This multi-layer will reflect the acoustic wave.
For these types of film bulk acoustic resonators, the resonant frequency is inversely proportional to the thickness of the piezoelectric layer. For example, in the case of an AlN resonator, a frequency of 1 GHz is obtained for a thickness of about 1 μm.
In RF applications, a search is conventionally made for resonant frequencies of the order of one gigahertz, corresponding to piezoelectric material thicknesses approximately equal to one micrometer (usually 1 to several micrometers) with a control over this frequency within a few MHz or a few tens of MHz. This frequency variation corresponds to control over the thickness of the piezoelectric material layer of the resonator within less than 10 nm (corresponding to a dispersion of less than 1%). This thickness check is hardly possible in an industrial context.
Moreover, none of these components according to prior art is tuneable.
This invention overcomes this weakness in prior art by using an FBAR-MEMS type resonator (in other words a suspended film bulk acoustic resonator) with a system capable of applying a variable mechanical stress on the resonator.
Its purpose is a suspended film bulk acoustic micro-resonator comprising a beam made of a piezoelectric material fixed to a support and with a thickness selected to operate at a given resonant frequency, the beam being sandwiched in the direction of its thickness between excitation electrodes, characterised in that it also comprises means of modifying limiting conditions of the resonator composed of the excited beam in order to modify the said resonant frequency.
According to a first variant, the means of modifying the limiting conditions of the resonator comprise means of applying a mechanical stress on the beam.
According to a first embodiment, the means of applying a mechanical stress on the beam enable the application of an electrostatic force on the beam, resulting in bending of the beam. The beam may be fixed to the support by its two ends or by only one of its ends. If the beam is separated from the support by the presence of a cavity facing the beam, the means of applying an electrostatic force may include at least one electrode located on the said beam and at least one electrode facing this electrode and in the cavity. The electrode used to apply an electrostatic force located on the beam may be one of the excitation electrodes.
According to a second embodiment, the means of applying a mechanical stress on the beam are means of producing a bimetallic strip effect. The means of producing a bimetallic strip effect may include at least one bimetallic strip effect resulting from a metallic layer formed on one face of the beam and a heating element. They may then be placed on the same face of the beam. The means of producing a bimetallic strip effect can also form at least one double bimetallic strip, a first bimetallic strip effect device being located on one face of the beam and a second bimetallic strip effect device being located on the other face of the beam, these two bimetallic strip effect devices being located facing each other. The heating element may be an electrical resistance supported by the metallic layer and separated from the metallic layer by a layer of electrical insulation. It may also be composed of the metallic layer of the bimetallic strip effect device.
According to a third embodiment, the means of applying a mechanical stress on the beam are piezoelectric means. These piezoelectric means may include electrodes on at least one face of the beam, for applying an electrical field to the beam, which results in a compression/expansion stress or a shear stress in the plane of the beam. One of the electrodes of the piezoelectric means may be an excitation electrode.
According to a second variant, the means of modifying the limiting conditions of the resonator include means of applying a mechanical load on the beam. Advantageously, the means of applying a mechanical load on the beam include at least one loading beam fixed to the support and facing the beam made of a piezoelectric material so as to apply the said mechanical load under the action of an electrostatic force applied to the loading beam by means of applying an electrostatic force. These means of applying an electrostatic force may include a first electrode arranged on the loading beam and a second electrode consisting of one of the excitation electrodes.
According to a third variant, the means of modifying the limiting conditions of the resonator include means of modifying the temperature of the beam made of a piezoelectric material. Advantageously, the means used to modify the temperature of the beam made of piezoelectric material include at least one electrical resistance. The electrical resistance may be arranged on one of the excitation electrodes and may be separated from it by a layer of electrical insulation.
The beam made of a piezoelectric material provided with its excitation electrodes may be fixed to the support through a layer of a dielectric material.
The invention and its advantages and special features will be better understood after reading the following description given as a non-limitative example, with the appended figures wherein:
The following description applies to three variant embodiments of the invention.
According to the first variant, the limiting conditions of the resonator are modified by applying a mechanical stress to the beam forming the resonator. The mechanical stress may be the result of an electrostatic force causing bending of the beam, with or without modification to its end embedment. The mechanical stress may also be the result of a thermal bimetallic strip type effect causing pure compression effect (in the case of a double bimetallic strip) or a combined bending moment and compression (for a single bimetallic strip). The mechanical stress may also be the result of a piezoelectric stress introduced by excitation of the resonator itself (for example, deformation of the resonator in the direction of its length or its thickness).
According to the second variant, the resonator limiting conditions are modified by applying a variable load to the resonator. The variable load may originate from gluing located on an additional beam or a loading beam on the resonator through electrostatic forces.
According to the third variant, the limiting conditions of the resonator are modified by varying its temperature. The resonator temperature may be modified and controlled by means of a heating resistance located on the beam forming the resonator.
In the same was as for any mechanical resonator, this variation in the limiting conditions (stresses, load, temperature) is expressed as a variation in the resonant frequency of the system (modification to limiting conditions).
The possibility of controlling the resonant frequency of oscillators according to the invention means firstly that control over the thickness of the layer of piezoelectric material can be relaxed, and secondly the resonance of each of the oscillators can be controlled dynamically in order to make a tuneable filter.
A layer of piezoelectric material 1 is suspended above a cavity 2 made in a support 3. It is of the built-in beam type at both of its ends. For example, the depth of the cavity 2 may be between a few tenths and a few tens of μm, for example 1 to 2 μm. The piezoelectric material beam 1 is sandwiched between a first set of electrodes; an upper electrode 4 and a lower electrode 5. This first set of electrodes is intended to apply one or more resonant modes to the beam. This is achieved by connecting electrodes 4 and 5 to an alternating voltage generator 6 at the resonant frequency of the chosen mode.
A second set of electrodes is used to apply an electrostatic force to the beam. This second set of electrodes may be composed of an electrode 7 placed at the bottom of the cavity 2 and by the lower electrode 5 arranged facing the electrode 7. A polarisation voltage output by a DC current generator 8 is applied between the electrodes 5 and 7 to induce bending of the beam. This bending generates a stress on the resonator, which is translated by a change to its resonant frequency. Since the stress is a function of the polarisation voltage, the resonant frequency is also related to the polarisation voltage.
For technological production reasons, a layer 9 forming the support beam may be inserted between the support 3 and the resonator. The layer 9 may be made of silicon nitride or oxide.
Deformation of the beam 1 under the effect of an electrostatic force has been shown as dashed lines.
A beam made of a piezoelectric material 91 is suspended as a cantilever above a cavity or a recess 92 in a support 93. The beam 91 is sandwiched between an upper electrode 94 and a lower electrode 95 that will apply one or more resonant modes to the beam. To achieve this, the electrodes 94 and 95 are connected to an alternating voltage generator 96 at the resonant frequency of the chosen mode.
An electrode 97 is placed at the bottom of the cavity 92 and is coated by a layer of dielectric material 99. A polarisation voltage output by a DC voltage generator 98 is applied between the electrodes 95 and 97 to induce bending of the beam.
A thermal bimetallic strip may be arranged on one or several parts of the beam, on or underneath the beam. In the case shown in
By passing a current in the electrical resistance of a heating element, the temperature of the bimetallic strip (composed of the layer with a high coefficient of thermal expansion and the beam) increases. Under the effect of this temperature increase, the differential expansion of the two materials of the bimetallic strip generates a compression force and a bending moment on the beam. If the beam supports two bimetallic strips arranged as shown in
Additional electrodes on the top face and/or the bottom face of one or several parts of the beam are arranged to generate a continuous electric field in the beam made of a piezoelectric material 61. The arrangement of these electrodes depends on the orientation of the piezoelectric crystal and the required deformation direction. In the case shown in
A beam made of a piezoelectric material 121 is suspended above a cavity 122 formed in a support 123. The cavity 122 may be 1 or 2 μn deep. The beam made of a piezoelectric material 121 is sandwiched between an upper electrode 124 and a lower electrode 125 that will excite the beam. To achieve this, the electrodes 124 and 125 are connected to an alternating voltage generator 126 at the resonant frequency of the chosen mode.
The assembly composed of the beam 121 and the electrodes 124 and 125 is fixed to the support 123 through elements 127, 129, 120 and 130. Element 127 is an electrode fixed to the support 123 and overhanging the cavity 122. It is coated by a layer of dielectric material 129. Elements 120 and 130 are spacers that maintain a spacing between the set of elements 125-121-124 and the set of elements 127-129 when the voltage generator 128 does not apply a DC voltage between electrodes 125 and 127, thus forming a loading beam. This is shown in
A beam made of a piezoelectric material 141 is suspended above a cavity 142 made in a support 143. The cavity 142 may be between 1 and 2 μm deep. The beam made of a piezoelectric material 141 is sandwiched between an upper electrode 144 and a lower electrode 145 that will excite the beam. To achieve this, the electrodes 144 and 145 are connected to an alternating voltage generator 146 at the resonant frequency of the chosen mode.
Unlike the micro-resonator shown in
In the case described above, the counter electrode used to deflect the loading beams is the upper electrode of the beam made of a piezoelectric material. However, a counter electrode distinct from this upper electrode could be used. This is shown in
A beam made of a piezoelectric material 41 is suspended above a cavity 42 made in a support 43. The cavity 42 may be 1 or 2 μm deep. The beam made of a piezoelectric material 41 is sandwiched between an upper electrode 44 and a lower electrode 45 that will be used to excite the beam. To achieve this, the electrodes 44 and 45 are connected to an alternating voltage generator 46 at the resonant frequency of the chosen mode.
The excitation electrode 44 supports a sequence consisting of a layer of dielectric material 49 acting as an electrical insulation and a filiform heating resistance 47. The temperature increase induced by passing an electrical current in the resistance 47 modifies the characteristics of the resonator, which results in a variation of the resonant frequency.
In the different variants of the micro-resonator according to the invention described above, the beam is separated from the support by the presence of a cavity. Another approach would be to etch the entire back face of the support to release the beam.
The layer 9 forming the support beam visible in
We will now describe the manufacture of a micro-resonator according to the invention. This embodiment is illustrated in
A 1.5 μm deep cavity 102 is formed in the silicon oxide layer 103. This is shown in
An 0.8 μm thick metallic deposit is made at the bottom of the cavity 102 to supply the electrode 107 that will deform and constrain the resonator, as shown in
A sacrificial layer 110 is then deposited in the remaining part of the cavity to fill it and to reach the top level of the layer 103. This is shown in
A 0.2 μm thick metallic deposit is made in order to make the lower excitation electrode 105 of the resonator. This is shown in
The resonator beam made of a piezoelectric material 101 is then deposited on the lower electrode 105. This is shown in
A 0.2 μm thick metallic deposit is then made on the beam made of a piezoelectric material 101 to obtain the upper excitation electrode 104 of the resonator. This is shown in
The next step is then to remove the sacrificial layer 110 to release the resonator and obtain the structure shown in
Therefore, this invention can be used to make integrated and tuneable RF resonators and filters using a manufacturing process compatible with manufacturing of integrated circuits. It can be used to make more “robust” filters since the resonant frequency of each oscillator can be adjusted after it has been manufactured, by controlling the mechanical stress and no longer by a very precise control over the thickness of the beam made of a piezoelectric material. It makes it possible to make a tuneable filter.
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|U.S. Classification||310/330, 310/320, 310/331|
|International Classification||H03H9/17, H01L41/08, H03H9/24, B81B3/00|
|Cooperative Classification||H03H9/172, H03H2009/241, H03H9/173|
|European Classification||H03H9/17A1A, H03H9/17A1|
|Feb 28, 2005||AS||Assignment|
Owner name: COMMISSARIAT A L ENERGIE ATOMIQUE, FRANCE
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:ROBERT, PHILIPPE;REEL/FRAME:016321/0759
Effective date: 20040803
|Feb 16, 2013||FPAY||Fee payment|
Year of fee payment: 4